The present invention relates to a method and apparatus for removing suspended matter from liquid. The method and apparatus of the present invention further relates to liquid-solids and/or liquid-liquid separation. More particularly, the present invention relates to a method and apparatus which coalesces and separates suspended matter from water.
Multiple industries including oil, paper and pulp, textiles, electricity generating and food processing present an ever present and growing problem of contaminated liquids, especially water as a by product of the various processes; in particular, water produced in the oil and gas industry. A method commonly used to separate oil and suspended material from water and other liquids is the air flotation separator. Two basic types of gas flotation systems exist, the dissolved air or gas and the induced air or gas approach. Both systems disperse gas into the contaminated liquid thereby causing the formation of bubbles which adhere to the suspended materials, thereby floating the suspended materials to the surface of the liquid for removal. Dissolved air systems disperse a relatively small gas volume and produce a very small gas bubble, for example about 100 microns or less. The small bubbles have massive surface area potential for impurity adhesion. Due to the relatively small gas volumes employed however, known apparatus require large retention tanks to facilitate the mass transfer of the low gas rate microscopic bubbles into the impurity laden water. As a result, dissolved gas flotation is impractical because of the size of the equipment, expense of construction and more importantly, space limitations on oil producing platforms.
Hydraulically induced gas separators draw larger gas volumes by recirculating water or the liquid back to the main separator. The gas entrained recycled water is dispersed within compartmented aeration cells. The objective of these systems is to contact the water passing through the cells with gas bubbles. While the hydraulic flotation devices are generally smaller in size than the dissolved air systems, the hydraulic flotation devices present a new series of problems For example, the larger bubbles formed by the higher gas rates create turbulence within the shallow depth aeration chambers. Floated impurities which may otherwise collect on the surface are consequently reentrained within the liquid. Mechanical induced flotation utilizing motorized impellers to draw gas are also plagued with a number of fabrication, operational and maintenance disadvantages. One major disadvantage of both the hydraulic and mechanical flotation cell is the system tank design. Due to cost restraints, the substantially rectangular tank cannot withstand internal pressure of more than just a few ounces per square inch. In view of existing and proposed government environmental and safety regulations, leaking access covers and vents to the atmosphere are unacceptable. Hazardous emissions, particularly lethal hydrogen sulfide and the like, exposure to operating personnel or the environment are of particular concern when clarifying water which has been generated in the oil industry, i.e., produced water.
U.S. Pat. No. 4,564,457 provides another gas induced flotation separator apparatus. The flotation apparatus teaches improvements to the induced gas flotation methodology by providing a tank which can be built to withstand the internal pressures common in the petroleum production and chemical manufacturing industries. The 457 patent also teaches a smaller sized tank with no moving parts. The apparatus comprises a cylindrical tank having an inlet chamber, a plurality of gassification chambers and a quiescent outlet chamber A skim trough is disposed near the top of the tank and extends the length of the gassification chambers into the outlet chamber Vertical baffles which separate the individual chambers extend downwardly and are spaced from the bottom of the tank, allowing fluid to flow along the bottom of the tank from the inlet chamber to the outlet chamber. Each gassification chamber is equipped with an eductor nozzle assembly positioned centrally in the lower portion thereof. The nozzle assembly provides for recirculation of fluid pumped from the outlet chamber.
Yet another system disclosed in U.S. Pat. No. 4,255,262 provides an apparatus which mixes and disperses gas in the form of fine bubbles in liquid in a tank in an attempt to remove contaminants from the liquid flowing through the tank. The gas is induced from an upper section of the tank downward into the liquid in the tank via a draft tube. The gas induction occurs as a portion of the liquid contained in the vessel is recirculated back through the individual cells or compartments using a centrifical pump. The apparatus uses a mechanical skimmer assembly which serves to remove contaminant laden froth as the froth accumulates above the liquid level section of the tank.
The aforementioned apparatus suffers from various deficiencies; for example, the skimmers are moderate to high maintenance items especially in corrosive environments frequently encountered in the oil producing or chemical industries In addition, the tanks are rectangular and cannot withstand pressure in excess of a few ounces per square inch internally. Such pressure limitations are particularly disadvantageous especially where the system pressure upstream of, for example, an oil/water separator, is prevalent or where noxious or lethal gases such as hydrogen sulfide are present Furthermore, the rectangular tanks having the sideboard skimmers are limited in volumetric capacity because full utilization of the tanks is not allowed. Although these tanks are described as "gas tight", low gas pressures are maintained by continuously venting gas to the atmosphere which is a potentially dangerous practice if lethal or flammable gasses are present and may be in violation of federal, state and local environmental requirements.
While the features of U.S. Pat. No. 4,564,457 present improvements to conventional flotation separation technologies, the patent fails to envision a single pass flotation separator system which has the ability to create microscopic bubbles with the higher gas volume necessary for optimum mass transfer of the gas to the impurity laden liquid and to do so without the consequences of inter-cell turbulence. Particularly, the patent does not envision the aforementioned improvements of the flotation separator technologies utitlizing a single flotation chamber. The reference also fails to provide an induced gas flotation cell which has external gas rate adjustability; therefore the need still exists to drain the liquid from the tank, purge the vapors, and then physically enter the unit for adjustment. The 457 patent also neglects small bubble diameter in the presence of high gas rates which promotes maximum contact-coalescence. As a result, systems manufactured and utilized as taught by the aforementioned patent and other conventional designs, remain physically large utilizing multiple aeration cells in order to achieve the efficiency of liquid clarification achievable by the single cell apparatus of the present invention. The present invention provides apparatus and methodology for inducing gas rates substantially greater than heretofore applied in the known flotation art, but without the inter-cell turbulence and reentrainment consequences observed in the conventional designs. The present invention provides apparatus comprised of a reduced equipment sizing, yet having the ability to generate microscopic bubbles even in substantially flat water i.e., tap water. The apparatus further provides a relatively quiescent surface with minimum wave generation. In addition, high free oil surges are sensed and diverted to skim means thereby avoiding noncontaminant clear water effluent.
In yet another embodiment, the apparatus and method according to the invention allows for utilizing a modified, existing vessel in combination with an exterior skid mounted induced gas liquid coalescer. In one mode of operation, the apparatus and method provides for recycling of gasified clarified liquid from the separation vessel and, in combination with a relatively larger system including the separation vessel, the method utilizes gas bubble mass transfer within the retrofitted vessel to accomplish the contact-coalescense surface area required for adequate separation.
In another mode of operation, the apparatus and method according to the invention allows for the utilization of influent feed flow from a primary collection and skimmer vessel with gasified discharge from invention apparatus into a second final clarification vessel wherein oil and solids laden froth is skimmed from the liquid surface. Clarified water discharge from second vessel being adequate for disposal and/or further polishing.